1. Field
The present invention relates to the field of photolithography to form integrated circuits and more particularly to the field of photoresists used in photolithography.
2. Discussion of Related Art
Photolithography is used in the field of integrated circuit processing to form the patterns that will make up the features of an integrated circuit. A photoresist is employed as a sacrificial layer to transfer a pattern to the underlying substrate. This pattern may be used a template for etching or implanting the substrate. Patterns are typically created in the photoresist by exposing the photoresist to radiation through a mask. The radiation may be visible light, ultraviolet (UV) light (which includes “broad band”, “g-line”, and “i-line”), deep ultraviolet light (DUV), extreme ultraviolet (EUV) light, or an electron beam. In the case of a “direct write” electron beam, a mask is not necessary because the features may be drawn directly into the photoresist. Most photolithography is done using either the “i-line” method or the chemical amplification (CA) method. In the i-line method components of the photoresist become directly soluble when irradiated and may be removed by a developer. In the chemical amplification method the radiation applied to the photoresist causes the decomposition of a photo-acid generator (PAG) that causes the generation of a small amount of acid throughout the resist. The acid in turn causes a cascade of chemical reactions either instantly or in a post-exposure bake that increase the solubility of the resist such that the resist may be removed by a developer. An advantage of using the CA method is that the chemical reactions are catalytic and therefore the acid is regenerated afterwards and may be reused, thereby decreasing the amount of radiation required for the reactions making it possible to use shorter wavelengths of light such as EUV. The photoresist may be positive tone or negative tone. In a positive tone photoresist the area exposed to the radiation will define the area where the photoresist will be removed. In a negative tone photoresist the area that is not exposed to the radiation will define the area where the photoresist will be removed. The CA method may be used with either a positive tone photoresist or a negative tone photoresist.
The modulation of the performance of a photoresist is controlled within a photoresist formulation by the choice of photo-acid generator (PAG), the control of polymer molecular weight, the polymer primary structure, molecular weight distribution, polymer side group's structure, species relative ratios, and the solvent system.